Here's why:
* Gerund phrases start with a gerund (the "-ing" form of a verb used as a noun) and include any modifiers or objects that go with it. For example, "Playing the piano is my favorite hobby."
* "Brushing hair" is a present participle phrase used as a noun. It acts as the direct object of the verb "is." It doesn't function as a verb in the sentence.
Here's a corrected example:
* Brushing hair is important for its health. (Present participle phrase used as a noun)
Let me know if you have any other grammar questions!